PETERS. — METABOLISM AND DIVISION IN PROTOZOA. 489 



The above explanation may be considered only as a possibility, and as 

 showing that there is no necessary inconsistency in the previous expla- 

 nation of the results with distilled water on the one hand, and on the 

 other, single salts at higher concentration and milk sugar. These consid- 

 erations further emphasize the importance of permeability in some funda- 

 mental physiological problems. 



We pass finally to the middle, or approximately isotonic, region of the 

 curves. As these concentrations are approached from either direction it 

 is observable that the values of the mean results tend upward toward 

 unity, or even to a point above that. It is true that the curves given lack 

 a series of values between those afforded by distilled water and by the 

 isotonic region. Data in this hypisotonic range are given in the next 

 following section. Yet it is plain that the addition of even a single physio- 

 logical salt to distilled water produces a rise in the curve that may cause 

 it to reach unity in approximately the isotonic region, as is here actually 

 shown, or possibly even sooner. The corresponding conditions have 

 already been considered to some extent in describing the osmotic relations 

 in normal or native culture media. Upon the previously applied princi- 

 ples of permeability, partial pressures, and the physiological conditions 

 of the cell-wall in the particular experiment, it is not difficult to construct 

 a picture of what probably occurs. It is not necessary to restate these 

 details, but I desire to add the following considerations. The metabolism 

 of the cell will cause the partial pressures of the physiological salts inside 

 the cell to undergo continual and probably unequal fluctuations for the 

 different salts. In other words, anabolic and katabolic changes will 

 result in a continually varying number of salt molecules, and a conse- 

 quent equilibration of partial pressures, which is, in all probability, 

 unequal for the different individual salts. A slight change in the compo- 

 sition of the external medium would also entail a rearrangement of 

 osmotic pressures. Complete osmotic equilibrium, that is, complete 

 isotonicity, probably never exists at any time during the life of the 

 animal. These continual changes are part of the normal, vital metabo- 

 lism of the cell. They show the deep significance to the organism of 

 what seems at first to be merely the fulfilment of the necessities of 

 physical law. 



The conditions cannot, of course, be entirely normal as long as we 

 apply externally only a single salt. But with even a single salt the 

 approach towards normal conditions becomes evident by a rise in the 

 curve under the following conditions. As the concentration increases 

 from the hypisotonic towards the isotonic region, or as the concentration 



